FASUDIR will provide an Integrated Decision Support Tool (IDST) based on a new methodology supported by a software tool that will help decision makers to select the best energy retrofitting strategy to increase the sustainability of the whole district. FASUDIR's IDST will incorporate key actors and relevant networks at the district scale and the entire value chain, and will ensure that new technologies for energy and resource efficient retrofitting are incorporated, especially those emerging from SMEs. FASUDIR takes advantage of the economies of scales and the variety of stakeholders when scaling up to the district, giving more opportunities for the development of new business and financial models with high impact in the economy. The developed methodology and software tool will be validated in three diverse urban areas, including a variety of historical periods.